The invention relates to a carbon monoxide filter that can be used, for example, in a fuel cell system.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a polymer electrolyte membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas comes into contact with and passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas comes into contact with and passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
The invention relates to a carbon monoxide filter that can be used, for example, in a fuel cell system.
Under some circumstances, carbon monoxide is present in the anode gas and/or the cathode gas. Without a carbon monoxide filter, the carbon monoxide can adsorb to a catalyst layer of a fuel cell, thereby blocking sites for hydrogen adsorption and/or oxidation on the catalyst layer. This can reduce the performance of a fuel cell or a fuel cell stack.
The carbon monoxide filter includes an electrochemical cell that can be switched between an adsorbing potential and an oxidizing potential. The carbon monoxide filter can be disposed along the flow path of the gas such that the gas contacts the carbon monoxide filter layer before contacting the catalyst layer. When the cell is at the adsorbing potential, at least some of the carbon monoxide present in the gas can adsorb to the carbon monoxide filter, which reduces the amount of carbon monoxide that is available to adsorb to the catalyst layer, thereby improving performance of the fuel cell or fuel cell stack.
After adsorption, the filter can be regenerated or reactivated by switching the potential to the oxidizing potential, which oxidizes the adsorbed carbon monoxide to carbon dioxide. After oxidation, the cell is switched back to the adsorbing potential, which again reduces the amount of carbon monoxide in the gas by allowing the CO to adsorb on the filter. Thus, by cycling the potential of the filter between an adsorbing potential and an oxidizing potential, the amount of the CO in the gas is effectively minimized. The carbon monoxide filter can have a relatively compact, simple and economical design.
In one aspect, the invention features a fuel cell system that includes a fuel gas supply, a fuel cell having an electrode in fluid communication with the fuel gas supply, an electrochemical cell, and a device. The electrochemical cell includes a cathode, an anode in fluid communication with the fuel gas supply and the electrode of the fuel cell, and an electrolyte in electrical communication with the cathode and the anode. The device is in electrical communication with the anode and the cathode, and is adapted to vary the potential of the anode relative to the cathode.
In another aspect, the invention features a fuel cell system that includes a fuel cell having an electrode, an electrochemical cell, and a device. The electrochemical cell includes a cathode, an anode in fluid communication with the electrode of the fuel cell, and an electrolyte in electrical communication with the cathode and the anode. The device is in electrical communication with the anode of the electrochemical cell, and is adapted to vary the potential of the anode.
The electrochemical cell can be arranged as a membrane electrode assembly, for example, one having a first catalyst layer composing the cathode, a second catalyst layer composing the anode, and a solid electrolyte between the first and second layers. The electrolyte can include a solid polymer, such as one having sulfonic acid groups. The anode or second layer can include a material selected from a group consisting of ruthenium, molybdenum, and iridium. The cathode is capable of serving as a reference hydrogen electrode.
The electrochemical cell can further include a first gas diffusion layer and a second gas diffusion layer, wherein the first catalyst layer is between the first gas diffusion layer and the solid electrolyte, and the second catalyst layer is between the second gas diffusion layer and the solid electrolyte.
The electrochemical cell can be between the fuel cell and the fuel gas supply, such as a reformer capable of producing a gas comprising hydrogen. The fuel cell can be between the fuel gas supply and the electrochemical cell. The fuel supply system can be in fluid communication with the anode.
The device is adapted to control the potential of the anode relative to the potential of the cathode.
In some embodiments, the fuel cell system further includes a second electrochemical cell having a cathode, an anode in fluid communication with the electrode of the fuel cell, and an electrolyte in electrical communication with the cathode and the anode. The cathode of the first electrochemical cell can be in fluid communication with the cathode of the second electrochemical cell.
The fuel cell system can further include a mixing chamber in fluid communication with the anode and the electrode of the fuel cell.
In another aspect, the invention features a method of treating a gas flow in a fuel cell system. The method includes contacting an anode of an electrochemical cell with an inlet gas stream, and changing the potential of the anode.
Changing the potential of the anode can include cycling the potential between a first potential and a second potential, e.g., as a function of time or a detected current.
The first potential can be at a level sufficient for carbon monoxide to adsorb to the anode, such as about zero relative to a reference hydrogen electrode, and the second potential can be at a level sufficient for the anode to oxidize the adsorbed carbon monoxide. The potential of the anode can be changed relative to a potential of a cathode of the electrochemical cell.
The method can further include contacting the inlet gas stream with an anode of a second electrochemical cell and/or mixing the inlet gas stream.
Other features, objects, and advantages of the invention will be apparent from the drawings, description, and claims.